Conversion of waste heat to electricity involves the steam-water Rankine cycle in most practical systems. The traditional steam power plant is based on any of a variety of fuels including nuclear, coal, oil, wood, etc. and, along with hydroelectric installations, has been the backbone of the electrical power-grid of North America
Steam systems have a number of advantages. Water (steam) is readily available and environmentally benign. Water has a large enthalpy change over typical pressure ranges. The Rankine cycle operates at temperatures and pressures that are fairly convenient. There are many competitive suppliers of equipment. Finally, the knowledge of owners, engineers, operators and maintenance personnel is well developed.
Steam systems have a number of disadvantages. Water has a tendency to erode, corrode and dissolve materials used in piping and equipment and contaminants accumulate in the re-circulating fluid. Water has an affinity to absorbing air that greatly degrades the system performance. Thus the boiler water must be treated chemically and continuously deaerated. For higher efficiency, most steam systems are operated in a vacuum at the heat rejection temperature. Air accumulates in the condenser and must be continually removed to maintain the vacuum and the low condensing temperature. Removing air is both an added equipment complexity and a parasitic energy load on the system. Also since the specific volume of low-pressure steam is very large, the condensing equipment can grow to enormous sizes. Operating requirements are legally mandated in most jurisdictions and require trained and skilled operators in constant attendance. Consequently steam systems become uneconomical in smaller power output sizes and when the heat source temperature is low.
Hydrocarbon fluids, most typically butanes and pentanes, have been used in geothermal power generating plants and similar applications where the heat source temperature is limited. These fluids operate similar to steam-water systems with the exception that they are closed systems and are under pressure at the heat rejection temperature. Such fluids are relatively expensive, flammable and environmentally sensitive. Their lower enthalpy characteristics require greater pressure ratios that need multi-stage turbines and greater flow rates that negate some of the equipment size reduction benefits of the positive pressure at rejection temperature. There are fewer suppliers and fewer knowledgeable operating and maintenance personnel available.
A related but different power cycle has been developed and patented by Alexander I. Kalina and is described in numerous patents; including U.S. Pat. No. 4,346,561, U.S. Pat. No. 4,489,563, U.S. Pat. No. 4,548,043, U.S. Pat. No. 4,586,340, U.S. Pat. No. 4,604,867, U.S. Pat. No. 4,732,005, U.S. Pat. No. 4,763,480, U.S. Pat. No. 4,899,545, U.S. Pat. No. 5,095,708, U.S. Pat. No. 5,103,899. The Kalina power cycle uses a mixture of water and ammonia for the purpose of increasing the energy conversion efficiency that can be obtained using the standard steam Rankine cycle. The cycle operates through a process of heating the binary fluid mixture, partially separating the components and applying the two fluid streams differently to enhance the overall efficiency of the power cycle. All the developments and teachings of Mr. Kalina build on this basic approach of component separation within the power cycle and differ from the present invention.